The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for rapid whole brain MRI.
In the last two decades, MRI techniques such as functional magnetic resonance imaging (“fMRI”) and diffusion weighted imaging (“DWI”), either for neuronal fiber tractography or for diffusion weighted diagnosis, such as in stroke, have revolutionized the ability of clinicians and scientists to investigate the human brain. These techniques mostly rely on echo planar imaging (“EPI”) or spiral sampling techniques for spatial encoding of the magnetic resonance image because of their fast scan times.
With contemporary scanner hardware, a single EPI or spiral image of a two-dimensional imaging slice can be obtained in tens of milliseconds and can be repeated at adjacent slice locations for coverage of a desired imaging volume, typically requiring on the order of 2-6 seconds for whole brain imaging, depending on the slice thickness and inter-slice gaps, if any. More specifically, an imaging slice of a desired thickness, such as a millimeter to a few millimeters, along one direction is excited with a frequency-selective radio frequency (“RF”) pulse. Subsequently, the image of this slice is encoded in the plane orthogonal to the slice direction with an EPI, spiral, gradient and spin echo (“GRASE”), fast or turbo spin echo, or other analogous ultra-fast imaging techniques, which use spatial encoding through phase modulation induced by magnetic field gradients. These approaches allow the plane perpendicular to the slice-selection direction to be spatially encoded after the single, slice-selective excitation. Thus, these methods are commonly referred to as “single-shot” techniques. In the aforementioned imaging techniques, slice excitation is followed by the formation of multiple echoes to generate a single echo train that encodes the two-dimensional image. The rapid two-dimensional image acquisition following the slice excitation is repeated in other locations so as to generate a “multi-slice” image of the entire desired imaging volume, such as the whole human brain.
Since its initial application, scan time volume coverage for ultra-fast imaging techniques, such as those described above, has not substantially decreased. For example, nearly all the successful efforts to shorten EPI acquisition times have targeted reducing the number of refocused echoes needed for spatial encoding to form an image, by means of method such as partial Fourier imaging, parallel imaging, or sparse data sampling techniques. Although these approaches decrease scan time for spatial encoding, they do not necessarily reduce the time required for image acquisitions by a significant amount. This is because a physiological contrast preparation period, such as for neuronal activity or water diffusion, must precede the spatial encoding period for each slice. Notably, this contrast preparation period can equal or exceed the time employed for collecting the EPI echo train.
Three-dimensional echo volume imaging (“EVI”) extends the EPI principle to three dimensions, and eliminates the separate slice excitations, thereby avoiding the need to repeat the contrast preparation period. Thus, EVI allows for a single contrast preparation period to be followed with a three-dimensional volume coverage in a single echo train. However, this approach has severe limitations in spatial resolution and image quality due to the longer echo trains needed to fully encode the volumetric spatial information in the relatively short acquisition period. Thus, these EVI techniques suffer from distortions and blurring on two of the three image axes, as well as a loss in signal-to-noise ratio (“SNR”). Multi-shot three-dimensional techniques, such as those based on EPI, that have produced high quality images overcome this limitation, albeit at the expense of longer acquisition times than can be achieved with EVI or single-shot three-dimensional GRASE sequences. Echo shifting approaches, such as principles of echo shifting using a Train of observations (“PRESTO”), increase volume coverage efficiency by taking advantage of echo time delays to apply additional RF pulses; however, these techniques are SNR limited and run into restrictions at higher magnetic fields when T2 and T2* become inherently short.
It would therefore be desirable to provide a method for rapidly imaging a significant imaging volume, such as the whole human brain, without the loss of spatial resolution and image quality, including signal-to-noise ratio, contrary to existing methods such as those mentioned above.